1. Field of the Invention
The invention is generally related to the field of fluid/fluid separation. More specifically, the invention relates to apparatus and methods for separation of moisture from fluids using a thermally regenerable material.
2. Related Art
Moisture, despite its necessity for sustaining life, is a prime enemy in many industries. The semiconductor fabrication industry is no exception. In fact, there are sophisticated heating (known in the semiconductor art as baking) methods used to dehydrate metal containers and tubing that are exposed to electronic specialty gases used to produce integrated circuits and other devices, such as flat panel displays. Baking is sometimes used in conjunction with granular beds, such as soda lime, sodium fluoride, heat activated aluminum oxide, finely divided nickel, or combination thereof. Soda lime (a mixture of calcium oxide with sodium hydroxide or potassium hydroxide), is typically used as an absorbent for carbon dioxide and water vapor. The use of granular bed such as soda lime, however, has the disadvantage of removing some of the usable gas. For example, HF is believed to react with calcium oxide to form calcium fluoride.
U.S. Pat. No. 5,030,251 discloses a system for separating a portion of a first gas (typically water vapor) from a gaseous feed (typically moist air) comprising a mixture of gases that is intermittently supplied to the system. While the patent is written in broad terms, the emphasis is placed on removal of water from air. According to the inventors of this patent, conventional membrane separators are quite effective during continuous (steady state) operation, but when the feed is supplied intermittently the system is often not as effective due to the fact that some residual water vapor remains in the separator when flow through the separator is stopped. When the flow is restarted, the residual water vapor flows out with the non-permeate product rather than permeating the membrane sidewalls. Thus, the non-permeate product exiting the separator during restart is not as dry as product exiting the separator during steady state operation. Therefore, in a system in which feed to the membrane separator is frequently cycled on and off, the membrane separator loses effectiveness. The system and method for treating feeds that are intermittently supplied described in the 251 patent comprises a membrane gas separator having at least one membrane through which the first gas permeates preferentially in comparison to other gases contained in the mixture. The separator is adapted to produce a non-permeate gaseous product which is discharged from a non-permeate gas side of the separator and in which the concentration of the first gas is lower than in the feed mixture. The system also includes a mechanism for supplying a gaseous purge stream to the separator when the feed mixture is not being supplied to the system. The concentration of the first gas is lower in the gaseous purge stream than in the feed mixture to purge residual amounts of the first gas contained in the membrane of the separator.
U.S. Pat. No. 5,160,511 discusses materials permeable to water vapor, including polymers of perfluoroethylene sulphonic acid that are available under the trade name “Nafion”, from E. I. DuPont de Nemours, which have the property of being freely permeable to the passage of water vapor but almost impermeable to nearly all other gases and vapors. Tubing and membranes made of this material are therefore used to add or remove water vapor from gas mixtures or from flowing stream of gas mixtures. The 511 patent notes one disadvantage of such a perfluoroethylene sulphonic acid polymer is that it is a very strong acid and somewhat unstable to heat, and resolves this problem by providing a membrane permeation unit comprising a perfluorocarbon polymer with lithium sulphate groups covalently bonded to at least a portion of the carbon skeleton, formed for example by immersing a polymer of perfluoroethylene sulphonic acid in a lithium hydroxide solution. A dehydrating unit utilizes this polymer as the separation means, as well as a dry gas (dry air) flowing on the permeate side of the membrane. There is no teaching or suggestion of means for regenerating the polymer, or use of these membranes in combination with other moisture removal materials.
U.S. Pat. No. 5,205,842 discloses a moisture removal system using a first or roughing stage to remove the bulk of moisture and a second or polishing stage to complete the drying. According to the 842 patent, the first stage can use a relatively small membrane area and a relatively low quality purge gas whereas the second stage can use a large membrane area and high quality purge gas, such as some product back purge. The 842 patent arrangement claims to provide efficient usage of available high quality purge gas, such as product, while minimizing product contamination, and the two stage purging arrangement is said to be especially attractive for those processes or systems where multigrade purge gases are available. The reference does not teach or suggest use of thermally regenerable moisture adsorption materials.
U.S. Pat. No. 5,641,337 discloses a process for the dehydration of a gas, particularly of a gas such as natural gas where the gas values should all be substantially recovered as dehydrated gas, in contrast to previous systems which wasted much of the usable gas. The process uses membrane dryers and provides a sweep gas for each of the dryers, where the sweep gas is substantially recovered and used again as a sweep gas. The process is stated to be economical for dehydration of large quantities of gas. Patents cited relating to dehydration of natural gas by use of membrane systems include U.S. Pat. Nos. 3,735,558; 4,497,640; and 4,718,921.
U.S. Pat. No. 6,709,487 discloses use of an adsorbent, method, and apparatus involving same for the removal of moisture from a fluoride-containing fluid such as gaseous nitrogen trifluoride. The adsorbent comprises an organic support such as polytetrafluoroethylene and at least one metal fluoride disposed within at least a portion of the organic substrate. The adsorbent may be regenerable or reused because the hydrated metal fluoride can be dissociated at an elevated temperature.
Despite improvements in the art, the need remains for apparatus and methods to efficiently and safely separate moisture from fluids, especially in the presence of fluids commonly used in the semiconductor and flat panel display fabrication industries. Fluids used in this industry include reactive gases such as HF, AsH3, HCl, NF3, O3, Cl2, Fl2, silane and its derivatives, as well as relatively inert compounds such as perfluorocompounds (PFCs), nitrogen, argon and fluids comprising a combination of moisture, inert and reactive gases. The need is especially acute when these fluids are required on a continuous basis by a semiconductor or flat panel display fabrication facility.